Electrical Submersible Pump Monitoring and Failure Prediction

ABSTRACT

Current supplied to electrical submersible pumps in wells is monitored, and signal processing based on wavelet analysis and phase diagram analysis is performed on the data obtained from monitoring. An incipient malfunction of the electrical submersible pump, such as one due to scale build-up in and around the pump, can be detected at an early stage.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional applicationSer. No. 13/712,444, filed Dec. 12, 2012, which claims priority to U.S.Provisional Application No. 61/570,030, filed Dec. 13, 2011, each ofwhich is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to monitoring performance and to failureprediction of electrical submersible pumps in wells.

2. Description of the Related Art

Submersible pumps have been used in wells for oil production at variousdepths and flow rates. The pumps are typically electrically powered andreferred to as Electrical Submersible Pumps (ESP's). ESP's were one ofseveral forms of what is known as artificial lift. ESP's were located intubing in the well and provided a relatively efficient form ofproduction.

An ESP system used in oil production included surface components at theproduction wellhead or platform and subsurface components located inproduction tubing or casing at the level of producing formations in thewell. Surface components included a motor controller and surface cablesand transformers for power transfer to the subsurface componentsdownhole. Subsurface components in the well included a pump, pump motor,fluid seals and power supply cables.

The downhole ESP pumps were immersed in the well fluids being pumped forproduction at the operating depths in the well and drove formationfluids to the surface with power supplied from the electrically poweredpump motor which received operating power from the surface over thepower supply cables.

During production from the formation, mineral deposits from theformation fluid occurred in and around the ESP's, well tubing and othersubsurface equipment, and have caused recurrent problems. The mineraldeposits were known as scale. One of the common failure reasons in ESPassemblies resulted from scale build-up in the pump stages, where scalegradually formed around the impeller vanes and eventually blocked fluidflow. Scale deposits led to a gradual decrease of the pump efficiencyuntil pump failure eventually occurred.

Problems with scale and other subsurface conditions as well as extendedservice eventually led to failure of the downhole ESP components,usually the pump. The causes and reasons of ESP component failure wereusually analyzed after the system had been pulled out or extracted fromthe well. The analysis commonly used after the ESP had been removed fromthe well was a detailed DIFA (Dismantle Inspection & Failure Analysis)process where each component of the ESP assembly was carefully analyzedfor an understanding of the nature of the failure. Experience has shownthat generally more than 20% of failure causes were attributed to motorfailure.

As noted, however, this form of failure analysis could only be performedafter the failure occurred, and after the downhole or subsurface ESPcomponents had been extracted from the well. Both the ESP failure andits removal from the well caused production from the well to be stopped.Production from the well was only resumed when a replacement ESPsubsurface system could be installed in the well. Production from thewell was thus interrupted for the time required for scheduling aworkover rig and its transport to the well, in addition to the time forinstallation of a replacement ESP subsurface system.

SUMMARY OF THE INVENTION

Briefly, the present invention provides a new and improved apparatus formonitoring performance of an electrical submersible pump in a well basedon analysis of pump electrical current. The apparatus according to thepresent invention includes an analyzer of the frequency spectrum of theenergy in the pump electrical current, and a wavelet analyzer of thewaveform of the pump electrical current identifying time variations ofthe pump electrical current. The apparatus also includes an analyzer ofthe pump electrical current to identify dynamic behavior of the pumpduring pumping, and a phase space analyzer forming a measure of theidentified dynamic behavior of the pump based on fluctuations in thepump electrical current. A graphical interface of the apparatus formsindications from the analyzers for monitoring performance of the pump todetect disturbances in performance of the pump.

The present invention also provides a new and improved method ofmonitoring performance of an electrical submersible pump in a well basedon analysis of pump electrical current. The frequency spectrum of theenergy in the pump electrical current is analyzed, and the waveform ofthe pump electrical current is analyzed to identify time variations ofthe pump electrical current. The pump electrical current is analyzed toidentify dynamic behavior of the pump during pumping, and a measure ofthe identified dynamic behavior of the pump is formed based onfluctuations in the pump electrical current. Indications of the resultsof the analysis are formed for monitoring performance of the pump todetect disturbances in performance of the pump.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an electrical submersible pump in awell.

FIG. 2 is an plot of an example log over time of motor current to anelectrical submersible pump.

FIG. 3 is a schematic diagram of diagnostic signal processing componentsaccording to the present invention for an electrical submersible pump.

FIG. 4A is a plot of example Fourier Transform plots from motor currentlogs for an electrical submersible pump.

FIG. 4B is a plot of example wavelet current plots from motor currentlogs for an electrical submersible pump.

FIG. 5 is an example phase space diagram obtained from processingaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an electrical submersible pump assembly P is shownin a well 10 at the location of a number of perforations 12 formed in acasing 14 to allow entry through an inlet or intake section 15 of oiland other hydrocarbon fluids from a formation 16 in a subsurfacereservoir. The casing 14 may also be a liner installed within largerdiameter casing in the well 10. A pump section 20 of the electricalsubmersible pump assembly P is immersed in the fluids in the casing 14.The electrical submersible pump assembly P is suspended within thecasing 14 on tubing 22 at the well depth of the perforations 12 so thatthe pump section 20 may drive or pump fluids in the casing 14 asindicated at 24 to a collection facility at the surface.

The pump section 20 includes a suitable number of centrifugal pumpstages which are driven by an alternating current pump motor 26. Thepump motor 26 receives operating electrical power over a cable 28 from asuitable power source 30 at the surface. The pump motor 26 drives ashaft that extends through suitable sealing for driving the centrifugalpump stages of pump section 20 of the electrical submersible pumpassembly P. The pump section 20 is conventional and comprises a largenumber of stages of impellers and diffusers.

The electrical submersible pump assembly P of FIG. 1 is equipped with amonitoring or logging system 32 to continuously record differentoperating parameters regarding the electrical submersible pump assemblyP to ensure the good functionality of both pumping system and associatedsensors. As part of the control and monitoring protocols of theelectrical submersible pump assembly P, the current waveform of theoperating power provided to the pump motor 26 over the cable 28 iscontinuously recorded along with other operating parameters, such as thefluid rates, the pump speed, intake and discharge pressures.

An example pump electrical current log of current amplitude as afunction of time is shown at waveform 40 in FIG. 2. The current logwaveform 40 illustrates an example of an ESP motor current over a periodof several weeks before a pump assembly failure as indicated at 42. Itcan be noticed from FIG. 2 that for a period of time of several daysbefore the failure, the pump current waveform 40 shows a substantialvariation in pump performance trends.

According to the present invention, a diagnostic processor 44 (FIG. 3)performs advanced signal analysis of pump motor operations to monitorfor the likely occurrence of a pump motor failure due to scale build-up.The signal analysis may be either as a computer-implemented method on ageneral purpose computer, or may be specifically configured digitalsignal processing circuitry or chips, or a combination of the two. Thediagnostic processor 44 processes the pump electrical current signallogs received from the logging system 32 after conditioning andconversion to a format for digital processing by a conditioning circuit45 (FIG. 3). The diagnostic processor 44 analyzes the pump electricalcurrent logs to identify the dynamical behavior and performance of thepump motor 26 (FIG. 1).

In accordance with the present invention, the diagnostic processor 44includes modules to analyze several aspects of variations recorded inthe motor current signals by logging system 32 to dynamically follow thepump operation and indicate events which indicate a likely occurrence offailure of the pump motor in advance of the actual failure. Time seriesof the motor current signals collected by the logging system 32 areanalyzed by Fourier transform analyzer module 46, a wavelet transformmodule 48 and phase attractor module 50, as shown in FIG. 3.

The Fourier transform analyzer module 46 is a processor which operateseither as a programmed digital signal processor or special purposeprocessing circuitry to analyze the pump current signal waveform byFourier analysis. The Fourier transform analyzer module 46 providemeasures of the power or energy present in the pump electrical currentas a function of frequency over its frequency spectrum. The Fouriertransform analyzer module 46 provides an indication of signal propertiesin a defined time window for the pump electrical current waveform.Fourier analysis by the analyzer module 46 yields an energy density inindividual frequency ranges of the power spectrum. The Fourier transformanalyzer module 46 in the preferred embodiment preferably performs asignal processing technique known as a Fast Fourier Transform. The powerspectrum obtained by a Fast Fourier Transform analysis allowsdetermination of the range of frequencies present in the pump electricalcurrent in the pump assembly P at times prior to a pump failure andidentifies characteristic frequencies of pump electrical current whenpresent.

FIG. 4A presents an example of the Fourier transform plots obtained fromthe module 46. In actual practice, plots like that of FIG. 4A are incolor to indicate in more detail, the data of interest. The Fouriertransform plot of FIG. 4A shows the characteristic frequencies of themotor current logs and exhibits amplitude peaks dominating the frequencyspectrum indicating a singular structure in the signal corresponding tothe scale build-up as an additional load on the ESP motor shaft.

The wavelet transform analyzer module 48 (FIG. 3) performs a waveletanalysis of the pump electrical current logs provided by the loggingsystem 32. The wavelet transform analyzer module 48 may also be aprocessor which operates either as a programmed digital signal processoror special purpose processing circuitry to analyze the pump currentsignal waveform by wavelet transform analysis.

The wavelet transform analyzer module 48 is a module determines signalcharacteristic variations of the pump electrical current waveform in thetime domain, while the Fourier analyzer module 46, as set forth above,analyzes signal characteristic variations of the pump electrical currentwaveform in the frequency domain. Wavelet analysis by the wavelettransform analyzer module 48 permits the tracking of the spatio-temporalevolution of the signal in various time scales.

The wavelet transform of a continuous signal s(t) representing the pumpelectrical current as a function time t is given by:

$\begin{matrix}{{C\left( {\tau,a} \right)} = {{- \frac{1}{\sqrt{a}}}{\int_{- \infty}^{+ \infty}{{\psi \left( \frac{t - \tau}{a} \right)}{s(t)}{dt}}}}} & (1)\end{matrix}$

where ψ is a mother wavelet, which is an absolutely integrable function.

Wavelet analysis in module 48 is performed by the dilatation andtranslation of the mother wavelet ψ. The parameter a in Equation (1) isrelated to the dilatation and is inversely proportional to frequency.Varying the parameter a for wavelet analysis in analyzer 48 changes thecenter frequency of the mother wavelet w and also the wavelet timeparameter. The parameter a is thus used rather than frequency torepresent the results of wavelet analysis in wavelet analyzer 48, aswill be seen.

The parameter τ is the translation or time-shift parameter. Theparameter τ specifies the location of the wavelet in time, andadjustment of the parameter τ causes the wavelet to shift over the pumpelectrical current signal being analyzed. For instance, a wavelet knownas the ‘Mexican hat function’ given by:

$\begin{matrix}{{\psi (t)} = {\left( {1 - t^{2}} \right){\exp \left( {- \frac{t^{2}}{2}} \right)}}} & (2)\end{matrix}$

may, for example, be chosen as the mother wavelet when the signal s(t)has high fluctuations. In the wavelet analyzer 48, a compression of theparameter a being varied to a lower value allows analysis of highfrequency components of the electrical pump current waveform, whilestretching of the parameter a to an increased value is related to lowfrequency components.

FIG. 4B is an example plot of the wavelet transform output formed by thewavelet analyzer module 48 as function of time-scale diagrams ofiso-correlation contours a/Δt of the parameter a for the parameter a fordifferent time shifts plotted in different frequency levels.

In the example of wavelet transform plot shown in FIG. 4B theiso-correlation contour plots are normalized against the highestcorrelation value to highlight the peak of the correlation values. Thecenters of contour zones corresponding to peak values clearly showperiodic structures appearing at different time scales. This periodicityin the peak alignments against the time scale exhibits the presence of aregular structure in the signal corresponding to an anomaly in thesignal pattern due to the scale build-up in the system resulting fromadditional load or torque on the ESP shaft. It can be seen also thatthese peaks have different a/Δt values indicating a progression in thedynamical behavior of the system, meaning progression of the scale loadon the rotating shaft.

The diagnostic processor 44 also includes a dynamic behavior analyzermodule 50 in which a time series signal corresponding to a certain flowregime of fluid through the pump assembly P is dynamically embedded inorder to determine the signal fractal dimensions that are used to buildthe dynamical attractor described in FIG. 5. Based on data from themotor current log obtained by the monitor 32, pump performance changescan be identified. The pump electrical current waveform is marked inbehavior analyzer module 50 by the superimposition in a module 51 ofseveral characteristic frequencies added in time. The presence of theadded time series signals allows, after return to steady state, thesignal identification of a resultant associated attractor indicatingpump performance. For example, the signal processing method of method ofmutual information can be used to estimate the time delay of each timeseries recorded to construct an attractor associated with eachintroduced time delay. The appropriate time delay is the one whichcorresponds to the first minimum value of the mutual informationfunction calculated from the time series.

The diagnostic processor 44 also includes a phase diagram reconstructionmodule 52, in which the dynamical behavior of the pump as indicated bymeasurements furnished by monitor 32 is determined from the embeddedsignals by reconstruction of phase diagrams. The signal embedding module51 and the phase diagram reconstruction module 52 may each also be aprocessor which operates either as a programmed digital signal processoror special purpose processing circuitry to identify dynamic behavior ofthe pump assembly P.

An example display of a reconstructed phase diagram formed by the module52 is shown in FIG. 5. Analysis of such data as that of FIG. 5 collectedfrom the motor current logs has shown that the fluctuations recorded hadvery low frequencies, which are repesentative of the gradual build-up ofthe scale in the different pump stages. The phase space reconstructionplot shown in FIG. 5 constructed from the signal recorded indicates aclearly typical chaotic tendency of the current logs shortly before pumpfailure. The plot in FIG. 5 exhibits clearly a typical attractor basinthat reveals from dynamical definitions the existence of a regularstructure in the signal linked to the additional load on the ESP motorshaft resulting from the gradual scale build-up. The combined plots fromFIGS. 4A, 4B, and 5 clearly highlight the changes in the ESP motorcurrent trends and dynamically indicate the existence of an additionalload on the motor shaft as the scale builds up in the different ESPstages.

The diagnostic processor 44 includes graphical interface 54 whichreceives processed data from each of the Fourier analyzer module 46, thewavelet analyzer module 48 and the phase diagram reconstruction module52 to form displays of the processed pump electrical current data. Thegraphical interface 54 forms displays of frequency spectra obtained formthe Fourier analyzer module 46 as shown in FIG. 4A. The graphicalinterface 54 also forms displays such as those shown in FIG. 4B based onprocessing results from the wavelet analyzer 48 and phase spacereconstruction diagrams shown in FIG. 5 based on processing results fromthe phase space diagram reconstruction module 52. The graphicalinterface is a user frendly environment that allows the user to displaythe plots desired from the above-mentioned methods and follow theevolution of the raw signal as well.

The graphical interface 54 provides as separate outputs the resultsformed in the diagnostic processor 44 as separate displays or windows.The analysis from the three displays provides indications to show anydisturbance present in the pump motor current logs and also providesadvance indications of pump performance or behavior likely to result inpump failure. The diagnostic processor 44 allows monitoring the growthof the scale on the electrical submersible pump assembly P byidentifying the magnitude of the disturbances in the motor current logs.

Diagnostic processing according to the present invention continuouslymonitors the performance of the electrical submersible pump assembly Pand predicts potential failure due to scale build-up. From motor currentrecorded prior to the failure, weak fluctuations in such current can berecorded, indicating a change in the motor load due to the scalebuild-up on the pump motor shaft. This scale build-up affects localizedshaft torque and therefore the total motor power draw. These changes canbe identified through the motor current draw. The advanced signalanalysis of the motor current data provided with the present inventioncan reveal the presence of a dynamical character changes of the pumpcurrent signal when scale starts rapidly building up in the pump stages.

Accordingly, the present invention provides a real time diagnosticsystem that predicts the likelihood of failure of an ESP system severaldays or weeks before the actual event of failure. This leads to bettercontrol of the well production protocol. For instance, if failure due toscale build-up is predicted through the diagnostic tools provided withthe present invention, several actions can be planned to prevent ordelay the pump failure. Such actions include, for example, reducing themotor speed to increase the production periods even at lower volumes.Thus, production may continue while a work-over rig is being scheduledfor replacement of the electrical submersible pump assembly, or whileplanning for an acidizing job to remove the scale. Such actions couldresult in avoidance of costly work-overs and minimizing lost productiondue to downtime.

The present invention thus identifies in real time disturbances in theperformance of the electrical submersible pump assembly. Detection canoccur at the very early stages of a pump motor malfunction resultingfrom change in the power draw due to scale build-up in the pump stages.Pro-active control of the pump run time can thus take place and remedialaction planned to prevent total pump failure. The present invention alsominimizes down time for pump repair in particular.

The invention has been sufficiently described so that a person withaverage knowledge in the matter may reproduce and obtain the resultsmentioned in the invention herein Nonetheless, any skilled person in thefield of technique, subject of the invention herein, may carry outmodifications not described in the request herein, to apply thesemodifications to a determined structure, or in the processingmethodology, requires the claimed matter in the following claims; suchstructures shall be covered within the scope of the invention.

It should be noted and understood that there can be improvements andmodifications made of the present invention described in detail abovewithout departing from the spirit or scope of the invention as set forthin the accompanying claims.

What is claimed is:
 1. An apparatus for monitoring and operating an electrical submersible pump motor disposed in a hydrocarbon well in situ to prolong operation of the pump motor and pumping of well fluids based on behavior and performance of the pump motor during pumping of the well fluids identified from an analysis of a pump motor electrical current providing operating electrical power to the pump motor, the apparatus comprising: a logging system recording pump motor electrical current logs of the operating electrical power provided by the pump motor electrical current to the pump motor during pumping of the well fluids; a conditioning circuit receiving the pump motor electrical current logs and converting the pump motor electrical current logs to a format for processing; a frequency spectrum analyzer receiving the pump motor electrical current logs from the conditioning circuit and providing measures of energy density present during pumping of the well fluids in individual frequency ranges of a frequency spectrum of the pump motor electrical current; a wavelet analyzer receiving the pump motor electrical current logs from the conditioning circuit and providing measures of variations in signal time shift and frequency characteristics of the pump motor electrical current during pumping of the well fluids to identify variations of the pump motor dynamic behavior as functions of time, the wavelet analyzer applying an adjustable wavelet to a pump motor electrical current waveform to identify additional load on the pump motor during pumping of the well fluids; and a graphical interface forming an output display of pump motor diagnostic information for monitoring the behavior and performance of the pump motor to detect disturbances in performance of the pump motor during pumping of the well fluids, the pump motor diagnostic information comprising: (a) characteristic frequencies and amplitude peaks as a function of frequency in the frequency spectrum of the pump motor electrical current measures provided by the frequency spectrum analyzer; and (b) variations of signal characteristics from the wavelet analyzer; and a diagnostic system configured to: predict, based on the pump motor diagnostic information, a failure of the pump motor; and reduce, in response to predicting the failure of the pump motor, an operating speed of the pump motor.
 2. The apparatus of claim 1, wherein the frequency spectrum analyzer comprises a Fast Fourier Transform analyzer.
 3. The apparatus of claim 1, wherein the wavelet analyzer applies the adjustable wavelet to the pump motor electrical current waveform and performs dilation and translation of the applied wavelet to identify variations in the pump motor electrical current.
 4. The apparatus of claim 1, wherein the additional load on the pump motor comprises scale build-up on a pump. 